Population ageing is an increasing median age in a population due to declining fertility rates and rising life expectancy. Most countries have rising life expectancy and an ageing population (trends that emerged first in developed countries, but which are now seen in virtually all developing countries). This is the case for every country in the world except the 18 countries designated as "demographic outliers" by the UN. The aged population is currently at its highest level in human history. The UN predicts the rate of population ageing in the twenty-first century will exceed that of the previous century. The number of people aged 60 years and over has tripled since 1950, reaching 600 million in 2000 and surpassing 700 million in 2006. It is projected that the combined senior and geriatric population will reach 2.1 billion by 2050. Countries vary significantly in terms of the degree and pace of ageing, and the UN expects populations that began ageing later will have less time to adapt to its implications.